[MAJOR] session: simplify buffer error handling

[MAJOR] session: simplify buffer error handling

Buffer errors (timeouts and I/O errors) were handled at two places,
just after the analysers and after again.

Now that the timeout detection has moved, it has become easier to
handle those errors.

This has also made it possible for the request and response analysers
to be processed together as a down-up event, and all the up-down I/O
updates to be processed afterwards, which is exactly what we're looking
for. Interestingly this has reduced the number of iterations of
(stream_int, req_resp) from (5,6,5) to (5,5,4).

Several tests have been run without any issue found.